Production of urban space is assumed to be a rent seeking activity, carried out by many developers who are constantly trying to get the best possible assessment of market and spatial features in order to better produce profit. Developers act locally, that is, they can choose their locations from all over the city, although they do not have a perfect foresight. Different urban locations prompt different types of development, e. g. several housing, commercial, industrial types, although each location can support more than one type development. The more structured the city is, the narrower is the type choice; on the contrary, in areas scarcely occupied the degree of freedom and unpredictability is larger. Developers compete to each other for locations where the land price is lower; in fact land is considered the single most important component of the development, in the sense that it is the only one whose price can vary and hence affect the price of the final good. Developers "invent" new locations constantly, by buying cheap land, investing on it and selling the resulting built space at higher prices. As soon as a developer discover a new location, others follow, making the land price to rise and the advantage to disappear. The resulting urban development is uneven and generally unpredictable; however, urban areas can be assessed from the point of view of its stability, or its resistance to change. A model has been tried out, as follows. Urban space is disaggregated in units of public space, each one presenting a certain level of urbanisation (infrastructure and built form), what affords a measure of spatial differentiation. Built form is also distinguished by their typological affiliation, so that the stocks are classified according to categories, precisely the ones taken on by developers. Moreover, stocks are also characterised by vintages, so that their actual market value can be updated after each iteration. The urban system is constituted by a network of public spaces, each one affording development of a certain number of types. This affordance is more restrict in the inner parts of the city and more open in the parts of scarce development or in sharp decay. The model tries to simulate the action of developers, either by giving them a profit goal or by fixing a urban growth preview. Decision making process is very simple; it involves choosing among different location alternatives the one which allies lowest costs (land and stocks to be devaluated) with highest values (more profitable building types). As developers compete and developments change the spatial affordances, dynamics of space production results unstable and shifting.